A lithium-ion rechargeable battery has advantages in that it is not only outstanding in energy density per unit volume, but also in energy density per unit weight, among all the battery systems in practical use at the present time. The energy density per unit volume is used as an index of reduction in size of a battery, and the energy density per unit weight is used as an index of reduction in weight of a battery. The energy density of a battery is determined mainly by the battery active material of the positive and negative electrodes, which constitute the power generation elements. However, reduction in size and weight of the battery case, which accommodates the power generation elements, also becomes important factors. In other words, when a battery case is made thin, it is possible to accommodate more battery active materials into a battery case with the same outside shape, and improve energy density per volume of the battery as a whole. It is also possible to reduce the weight of the battery as a whole, and improve energy density per weight, when the battery case is formed by a light material.
In the trend of the batteries mentioned above, a prismatic battery using a thin prismatic battery case as its outer case is regarded especially important, because it is suited in making an equipment thinner, and also because its efficiency in utilizing space is higher in comparison to cylindrical batteries. For conventional manufacturing methods of the prismatic battery case, there have been adopted the so-called transfer drawing method, which manufactures battery cases with substantially rectangular cross sectional shapes, by repeating deep drawing ten to twenty times with the transfer press machine. There have also been adopted a method that uses impact molding, using aluminum as its material.
However, in the manufacturing method of the prismatic battery case that uses the transfer drawing method, productivity is very low with only about twenty pieces per minute, for example, because the deep drawing has to be repeated ten to twenty times. Furthermore, it has a drawback in that the cost rises, because the die for multistage drawing becomes complicated, in addition to there being many steps. In the transfer drawing method, the thickness of the battery case material is made thinner by repeating the deep drawing, when making it thinner with object of making the capacity larger by raising the energy density per volume. Hence, a drawing-out and pushing-in of the punch is needed the same number of times as the number of drawings, and the diameter of the punch has to be made smaller each time, and the clearance between the punch and the die also has to be made smaller each time. Thus, it needs to be made the same thickness as the side face, from the thick portion at the circumference of the bottom part, and the deep drawing to fulfill this is very difficult. What is worse, the prismatic battery cases made from these processes lack strength at the circumferences of the bottom part, and there is a problem in that it is not possible to maintain a prescribed pressure resisting strength when functioning as a battery.
Japanese Patent Laid-Open Publication No. 2000-182573 discloses a prismatic battery case with its thickness of the plate portions on the longer sides configured to be thicker than the thickness of the corner portions. Japanese Patent Laid-Open Publication No. Hei. 6-52842 discloses a prismatic battery case with its thickness of the plate portions on the longer sides configured to be thicker than the thickness of the plate portions on the shorter sides. These prismatic battery cases are able to prevent expansion and deformation of the plate portions on the longer sides, when the pressure inside the battery rises. However, the capacity to accommodate the power generation elements becomes smaller, because the thickness of the plate portions on the longer sides are made to be thick. Since these portions have the largest surface area, it is not possible to pursue an improvement in energy density per volume or energy density per weight.
Japanese Patent Publication No. Hei. 7-99686 and Japanese Patent Laid-Open Publication No. Hei. 9-219180 disclose a battery case that pursues a reduction in internal resistance when functioning as a battery, by forming fine reinforcements vertical to the bottom face, inside the case, to pursue an increase in contacting area with the power generation elements. However, it is almost impossible to achieve a function that prevents expansion and deformation when the inner pressure of the battery rises, by using these vertical reinforcements of the battery case. Japanese Patent Laid-Open Publication No. Hei. 7-326331 discloses a prismatic battery case that has the thickness of the corner portion configured to be thicker than the thicknesses of the plate portion on the longer side and the plate portion on the shorter side, which are straight-line portions. This prismatic battery case can improve the efficiency to accommodate the power generation elements. However, it cannot prevent the expansion and deformation of the thin plate portions on the longer sides, being reinforced only by the thickened corner portion.
On the other hand, when manufacturing a prismatic battery case using impact molding, the prismatic battery case is formed by crushing a pellet, which serves as a battery case material, with a punch, and pressing and spreading the material out through the gap between the punch and the die, so that it spreads out along the outer peripheral surface of the punch. Productivity is improved in comparison to the transfer drawing method, but the precision of the dimensions is very bad, and the strength at its side portions will be insufficient when it is made thin. When a prismatic battery case is particularly functioning as a battery, and the pressure inside the battery rises, the deformation is bigger than that in a cylindrical battery case, which has a stable shape, and the plate portions on the longer sides with broad areas deform so as to expand into a cylindrical shape, which is a more stable shape. Consequently, there is a fear of the equipment being damaged by a leak of electrolyte solution, or by a short circuit of the power generation elements. For this reason, it is necessary to have a shape that reluctantly sacrifices reduction in thickness and weight, in order to maintain enough strength to positively prevent a deformation when the pressure inside the battery rises, in manufacturing a prismatic battery case by impact molding. Thus, it is not possible to pursue an improvement in energy density per volume or energy density per weight, in this case.
For other manufacturing methods of the prismatic battery case, Japanese Patent Laid-Open Publication No. Hei. 6-333541 discloses a method, in which a prismatic tube and a bottom plate are molded separately, and the bottom plate is joined hermetically to the bottom part of the prismatic tube by laser beam welding. However, in this manufacturing method, the number of steps are not reduced so much in comparison to the transfer drawing method, and troublesome operations intervene, such as the step that accurately positions the prismatic tube and the bottom plate, or the step that conducts the laser beam welding. Accordingly, it is not possible to pursue an improvement in productivity. Further, it is not possible to obtain a prismatic battery case that satisfies both of the contradicting requirements, high energy density by reduced thickness and weight, and pressure resisting strength that prevents deformation during a rise in inner pressure within the battery, in this manufacturing method.
A DI method is used for the manufacturing method of a battery case of a cylindrical battery. This DI method makes it possible to manufacture a battery case that maintains a prescribed pressure resisting strength, while making it possible to pursue an improvement in energy density per volume by reducing thickness. The DI method also makes it possible to manufacture with high productivity. This DI method is a method that conducts drawing and ironing on an intermediate cup body continuously, and in one action, and a battery case with a prescribed cylinder shape is manufactured using this method. This intermediate cup body is manufactured by deep drawing of a pressing machine. There are following advantages in this method compared to the transfer drawing method, and the extent of its use is enlarging. The advantages include an improvement in productivity with the reduction in the number of steps, an improvement in dimensional precision such as thickness, an improvement in energy density corresponding to the reduction in weight and the increase in capacity, which are both due to a reduction in thickness of the peripheral side walls of the case, and a reduction in stress corrosion.
Therefore, it is conceivable to manufacture a prismatic battery case using the DI method mentioned above. When manufacturing a cylindrical battery case using the DI method, it is processed between two geometrically similar shapes from an intermediate cup body having a cross section of circular shape to a battery case having a cross section of circular shape. The material flows uniformly during the processing, and deformation is done smoothly, because the thickness of the whole peripheral wall is reduced uniformly, in the ironing step of the DI processing. In contrast, when manufacturing a prismatic battery case using the DI processing, it is processed between two geometrically non-similar shapes from an intermediate cup body having a cross section of circular shape to a battery case having a cross section of substantially rectangular shape. This makes the flow of material non-uniform, and thickness deviation, shearing, and cracks all caused by eccentricity are easily induced. This also makes processing difficult in correspondence to the concentration of stress, because the processing stress acting during the molding is not uniform, and molding with high precision also becomes difficult, because it is not possible to conduct a stable processing. In particular, cracks and ruptures are liable to be induced at the plate portions on the shorter sides, which have small surface areas, and there arises a problem that portions with distorted shapes are generated.
Japanese Patent Laid-Open Publication No. Hei. 10-5906 also discloses a manufacturing method of a prismatic battery case. In this method, a first intermediate cup body is molded by drawing, and then a second intermediate cup body is molded by repeating the drawing on the peripheral side wall portion of the first intermediate cup body, plural times. Lastly, the second intermediate cup body is processed by impact extrusion (impact molding) to adjust the thickness to be a prescribed value at the bottom plate portion and the corner portion. However, the number of steps increases in this manufacturing method, with drawing, multi-stages of DI processing, and impact molding being necessary. Adjustment of the thickness at the bottom plate portion and the peripheral side wall portion also becomes difficult, because the thickness at the bottom plate portion is adjusted to be a prescribed value by impact molding in the last step. It is not possible to obtain a prismatic battery case having a shape with each of the portions conforming to prescribed thicknesses, with high precision. As opposed to this method, the present applicant has already proposed a manufacturing method that can manufacture a prismatic battery case that has high energy density, and a prescribed pressure resisting strength using the DI method. In the first step of this manufacturing method, a hoop material is punched and a battery case material 1 of oval shape is formed, as shown in FIG. 5A. Then, this battery case material 1 is processed by deep drawing to mold a first intermediate cup body 2 with a cross section of substantially elliptical shape similar to the circular shape, as shown in FIG. 5B. After that, the first intermediate cup body 2 is subjected to multi-stages of continuous redrawing of the second step, which uses drawing press machines. After being subjected to the second step, this first intermediate cup body 2 is molded into a second intermediate cup body 3, which has a cross section of substantially elliptical shape, and whose ratio of the minor-axis/major-axis is smaller than that of the cross sectional shape of the first intermediate cup body 2, as shown in FIG. 5C. Lastly, the second intermediate cup body 3 is subjected to the DI processing, which conducts drawing and ironing in a continued state, in the third step. This second intermediate cup body 3 is molded into a prismatic battery case 4, which has a shape with a cross section of substantially rectangular shape, and a shape that has the thickness at the plate portion on the shorter side 4a thicker than the thickness at the plate portion on the longer side 4b, as shown in FIG. 5D.
With this manufacturing method, it is possible to manufacture a prismatic battery case 4 of desired shape in three steps. Productivity is improved remarkably in comparison to the conventional transfer drawing method, and it is also possible to obtain a prismatic battery case 4 with high dimensional precision in thickness or the like by using the DI method. However, there are problems to be solved in this manufacturing method. That is, ruptures and cracks are induced, when attempting to manufacture a prismatic battery case by directly DI processing the first intermediate cup body 2. This is because an attempt is made to DI process the cup body 2 with a cross section of substantially elliptical shape similar to the circular shape into a case with a cross section of substantially rectangular shape. For this reason, an intervention of the second step is needed. In this second step, the dimension in the minor axis direction is gradually shortened by drawing, and the material of the deformation corresponding to this drawing is made to flow so that it escapes in the major axis direction. Moreover, the major axis side is modified by being shortened to a prescribed dimension. Consequently, the number of steps increases, because multi-stages of redrawing are to be conducted in the second step.
An object of the present invention is to provide a prismatic battery case having high energy density and a prescribed pressure resisting strength, and a prismatic battery using this prismatic battery case. It is also an object of the invention to provide a manufacturing method of the prismatic battery case, which makes it possible to obtain this prismatic battery case with high dimensional precision, and with ease, while reducing the number of steps by using the DI method and pursuing an improvement in productivity at the same time.